Corrosion resistant steel



Patented Ms fzs, 1939 of West Virg nia No Drawing.

.6 Claims.

rodeordinary iron and steel. Despite the excellent resistance of these chromium-nickelsteels tofuniformgeneral corrosion over the surface of the metal, undesirable deterioration and failure .occur from localized attack of saline solutions that exhibits itself in the form of holes or dei pressed areas, and in many important applications this localized attack renders an article use- .sumption has occurred.

Two distinct types of localized attack have been observed the effects of which are readily distinguishable although their causes maybe similar. One is characterized by the formation of \holesand pits distributed at random over the surface of metal subjected to corrosion although the actual area occupied by these small-holes constitutingthe article in question. This type of corrosion "is usually referred to as pitting.

0*The other is contact corrosion" which occurs fonly at and adjacent to an area of contact of some objectwith the corrosion-resisting metal "surface. This object may be of .the same matenickel steel. Although contact corrosion does not ordinarily proceed over the entire surface of the metal the diameters of the corroded areas are a'sa rule larger than those of the holes produced 40 by pitting; Frequently the affected areas resulting from contact corrosion contain numerous the tendency for the metal to pit.

Attempts have been made to overcome this 10- calized attack by increasing the percentages of wchromiumand nickel, thereby enhancing the taining even as much as chromium and 20% nickel are still subject to pit ting even though they are somewhat more resistant than the low- .er alloy steels to contact corrosion.

steel hasbeen decreased by lowering the chro- 55 mm and nickel contents to a point where genbinations of alloyed additions in relatively small less long before general. deterioration or coni or pits may represent only a 'fraction or the area I i "rial or it may constitute another-material such as dirt-oil, rust, glass, wood, or even chromiumsmeu w'ell defined pits indicating that contact corrosion in many instances actually accelerates general nobleness of the steel, but steels con- In the other direction, the noblenessof the UNITED STATES PATENT OFFICE coanosron nnsrsrm STEEL Russell Franks, Niagara Falls, N. Y., assignor to Electro Metallurgical Company, a corporation Application March 19, 1938, Serial No. 196,909

eral corrosion proceeds at a moderate rate, but this expedient makes the metal less resistant to contact corrosion and does not eliminate pitting.

I have tested the efiect of a great many all ed additions on the resistance of the austenitic '5 chromium-nickel steels to localized corrosion. Nearly all of those tested have exhibited no substantial beneficial effect. However, certain comproportions were found to improve the resistance to this formof corrosion to a surprisingly great degree. Briefly, molybdenum strongly retards pitting, and less strongly retards contact corrosion, and although manganese has. little efiect in inhibiting either form of attack, a mixture of 15 molybdenum and manganese greatly improves the resistance of the steels to both types of localized corrosion. The data given in Table A show the improvement secured by incorporagngrthese additions in the 18% chromium-9% nickel steels.

Tittle A Composition (remainder iron) Percent Percent Percent Percent Percent Percent Cr N1 Si Mn Mo 18 9 0. 09 0. 2 0. 6 None 18 9 0. 07 0. 5 0. 5 None 18 9 0.08 0.8 3 None 18 9 0.09 0.3 0. 5 2 18 9 0.10 0.3 0.5 3. 5 19 9 0. 08 v 0. 4 3. 1 2. 6

Condition of sample after- I Immersion in Immersion in Spraying with 20% N801 Solution 10% NaCl+0.5% 10% NaCl+ 5% for 1 eat F001; solution F001; solution y for 5 hours for 1.5 hours 40 land 2.-. Pitted and severe Badly pitted Badly pitted.

" contact corrosion in 5 months. 3 Slight pitting and do Do.

contact corrosion. 4....; do Unsfiected Unaffected. 5 Prfacgiclally unaf- Slightly p1ttod Slightly pitted.

ec e 6 "do Unaliected Unaflected.

' 'Ihistab le shows the results of tests made on different steels using sodium chloride solutions of different strengths with and without additions of ferric chloride. ,In making these tests, small cylindrical-shaped solid samples approximately V inch in diameter and 1 inch long were used. Each sample was given a relatively high polish on all surfaces so that attack could be easily detected. The salt spray tests made with a 20% sodium chloride solution consisted of exposing the sample to the spray from a 20% sodium chloride solution while one surface of the sample was in contact with a glass plate upon which it rested. At the end of a few months in this test .a steel sample readily susceptible to localized attack will become pitted on the top surface, whereas the bottom surface in contact with the glass will exhibit both pitting and contact corrosion. The tests made in the 10% sodium chloride solutions containing ferric chloride were employed to detect susceptibility to pitting alone.

It is shown in thistable that, at the end of a five-month period, the plain 18% chromium-9% nickel steels were found to be subject to both pitting and contact corrosion in the salt spray test, whereas in the sodium chloride solutions containing ferric chloride in which the samples were simply immersed, pitting occurred in relatively short periods. It is further shown in this table that steels containing molybdenum and low manganese are fairly resistant to pitting but during long periods of exposure gradually become subject to the contact type of attack. Other data in the table indicate that manganese additions in the neighborhood of 3% assist greatly in retarding the development of both pitting and con,- tact corrosion on exposure to the salt spray atmosphere but the chromium-nickel steels containing 3% manganese appear to be qlii'te susceptible to pitting in sodium chloride solutions containing ferric chloride. On the other hand, it is evident that by the incorporation of both molybdenum and manganese the steels are markedly improved with respect to both types of corrosion.

When suflicient manganese is incorporated, the effectis to oflset the tendency for the ferritic constituent to develop in the austenitic chromium-nickel steels as a result of the molybdenum addition, thereby permitting the manganesebearing steels to be rolled directly from ingots of large sizeinto articles of commerce. Without sumcient manganese present, the molybdenumbcaring chromium-nickel steels tend to check during the hot rolling or forging processes because of the excessive quantities of the ferritic constituent that is present in the cast chromiumnickel steels containing molybdenum. While it is true that the amount of the ferritic constituent can be reduced by increasing the carbon and nickel contents and reducing the chromium content of the steels, this results in decreased resistance to general over-all corrosion. Thus, the manganese performs another important function by permitting the fabrication of articles of low carbon content that also contain optimum percentages of chromium.

The invention comprises a novel article, such 4 as a container, which in normal use is subject to the corrosive action of saline solutions, that would (because of localized contact between the metal of the inside of the container and a solid object, or because of the presence of a small crevice in the container, or because the solution has a pH less than 7, or for more than one of these reasons) normally produce localized attack of an article composed of ordinary austenitic chromium-nickel steel, (18% chromium, 9% nickel, 0.1% carbon, less than 1% each of silicon and manganese, remainder iron). The novel article is composed of an austenitic chromium-nickel steel containing about 16% to 30% chromium,

num and less than 1.5% manganese. whic ticle is composed of an alloy steel'comprising' 16% about 7% to 20% nickel, carbon in amounts not exceeding 0.20%, and effective amounts of molybdenum and manganese that serveto inhibit the localized corrosive attack. The molybdenum is Per cent Chromium 16 to 24 Nickel 8 to 14 Carbon Not over 0.12 Molybdenum L. 2.50 to 4 Manganese 2 to 4 Silicon Not over 0.5 Iron The remainder The steel of which the container of the present invention is composed is adapted to fabrication byordinary methods, by forging and rolling, and

by welding, riveting, and similar joining means.

The steels are also soft, ductile and tough, possess excellent strength, and maintain their excellent corrosion resistant properties even 'after cold rolling if the cold work is applied uniformly to the section. One of the most important applications of the container is to hold and resist the action of sea water or water containing other chlorides such as exist in industrial applications.

Usually in these instances the corroding media have a pH value less than 7, a condition which causes pitting to take place readily in the plain chromium-nickel steels.

I claim:'

1. A container for saline solutions composed of an austenitic steel containing 16% to 30% chromium,j7% to 20% nickel, 2% to 5% molybdenum, 1.5% to 5% manganese, carbon in an amount less than 0.2%, silicon in an amount not exceeding 1%, the remainder iron; the molybdenum and manganese acting to impart to such container resistance to localized attack by saline solutions.

2. A container for saline solutions that normal 1y would produce localized attack of a container composed of an'austenitic chromium-nickel steel containing no molybdenum and less than 1.5% manganese, such container being resistant to the localized attack of saline solutions and being composed of an austenitic steel containing 16% to 24% chromium, 8% to 14% nickel, 2.5% to 4% molybdenum, 2% to 4% manganese, carbon in an amount not exceeding 0.12%, silicon in an amount not exceeding 0.5%, the remainder iron.

3. Article which in normal use is subjected to the prolonged action of saline solutions under conditions which tend to produce localized corrosion of the pitting and contact types in austenitic chromium-nickel steels containing no molybdearrosion.

4. Article which in normal use is subjected to the localized corrosive action of prolonged con- .analloy steel comprising 16% to 30% chromium,

tact with sea water, at least a portion of such water in contact with the article having a pH lessthan 7, which article is composed of a fully austenitic alloy steel comprising 16% to 24% chromium, 8% to 14% nickel, 2.5% to 4% molybde-fl .num,2% to 4% manganese, carbon in an amount conditions which, because of localizedcontact between said article and a solid object or because the solution has a pH less than 7, or both, tend to produce localiz'ed'corrosion of the pitting and contact types in' austenitic' chromium-nickel steels containing no molybdenum and less than 1.5% manganese,.which articles are composed of to said localized corrosive 9.0- f

7% to 20% nickel, 2% to 5% molybdenum, 1.5%

to 5% manganese, carbon in an amount not exceeding 0.2%, siliconin an amount not exceeding 1%, the remainder iron; the molybdenum and manganese acting to impart resistance to said 10- calized corrosion.

6. Articles which in normal use are subject to the localized corrosive action of prolonged contact with sea water having a pH less than '7, which articles are composed of an alloy steel comprising 16%"to 24% chromium,8% to 14% nickel, 2.5% to 4% molybdenum, 2% to 4% manganese, carbon in an amount not exceeding 0.12%, silicon in an amount not exceeding 0.5%, the remainder iron; the molybdenum and manganese acting to impart resistance to said localized corrosive action.

RUSSELL FRANKs. 

